Combustion processes, such as coal-fired boilers, produce flue gases that contain carbon dioxide which it may be desirable to capture and sequester, for instance in saline aquifers or in oil or gas wells where the carbon dioxide is used for enhancing the production of oil or gas from the well. However, flue gas often contains impurities such as SOx, NOx and Hg which must be removed down to very low levels before the carbon dioxide can be used for enhanced oil recovery or sequestered. The present invention is a process for achieving such removal to produce relatively pure carbon dioxide.